(a) Field of the Invention
The present invention relates to a process for producing high molecular weight acrylonitrile (hereinafter referred to as AN) polymers, and more particularly it relates to a process for producing high molecular weight AN polymers by suspension polymerization of monomer composed mainly of AN in an aqueous medium wherein particular polymerization conditions are combined and the polymerization in the water phase is suppressed.
(b) Description of the Prior Art
The molecular weight of AN polymers used for the production of acrylic synthetic fibers is generally from 50,000 to 200,000. The methods of industrial production of such polymers are divided broadly into two categories: aqueous polymerization of monomer composed mainly of AN in the presence of a water-soluble polymerization initiator in an aqueous medium, and solution polymerization wherein the monomer is polymerized in an inorganic or organic solvent which can dissolve AN polymers.
In the meantime, in recent years, a study was commenced for obtaining high strength fibers by dissolving a high molecular weight AN polymer in an organic or inorganic solvent, spinning the solution, and highly stretching the resulting fibers, as required. But it has not yet been successful in finding a useful method which permits the production of an AN having a molecular weight not less than 40,000, on a industrial scale and effectively.
On a laboratory scale, it is possible to obtain a high molecular weight AN polymer by bulk polymerization in which AN monomer is heated together with a polymerization initiator which is soluble in this monomer; or by photo solution polymerization in which AN monomer is polymerized by irradiation with ultra violet ray in an inorganic or organic solvent which can dissolve the resulting AN polymer. But in bulk polymerization, the operation of polymerization is complicated, and moreover there is a problem in that the polymer characteristics are not uniform. Also, in photo solution polymerization, the production cost is high, and there is a difficulty in the enlargement of the production apparatus and in the removal of impurities contained in the polymerization solution. Furthermore, the process involves various difficulties such that there is a rise of solution viscosity with the increase of molecular weight. Therefore, these processes cannot be evaluated as industrial means.
On the other hand, it may be possible to take into consideration a suspension polymerization process of a high molecular weight AN polymer in an aqueous medium, using an oil-soluble initiator and a dispersion stabilizer. But at usual polymerization temperatures about 7% AN dissolves in water, so that besides the suspension polymerization which occurs in AN monomer drops, a polymerization also takes place in the AN dissolved in the water phase, with the progress of polymerization. Moreover, the latter polymer formed in the water phase has a lower molecular weight and smaller particle diameters in comparison with the former polymer formed in the monomer phase. Therefore, the final product is a mixture of the two kinds of polymers formed in the monomer phase and water phase. Accordingly, in the case of this polymerization process, there is also a problem in producing a high molecular weight AN polymer having uniform molecular weight and particle diameters. In the polymerization on an industrial scale, it is common knowledge to polymerize at a high polymerization yield as possible, in order to heighten the productivity of the production process. However, polymerization at a high polymerization yield by suspension polymerization for obtaining a high molecular weight AN polymer results in mixing the low molecular weight polymer polymerized in the water phase, thus making it impossible to obtain a high molecular weight polymer only.